Telescopes and similar long range sensing devices are well known. Such devices typically comprise a primary objective lens, spaced apart from the main optical system and configured to collect photons and converge or otherwise guide and focus the collected electromagnetic radiation signals to, for example, a focal plane array (FPA), located on the focal plane, for sensing. Referring to FIGS. 1A, 1B and 1C of the drawings, many different types of telescope (or other long range sensor) exist.
Thus, referring first to FIG. 1A of the drawings, a refracting telescope (or other long range sensor) comprises a converging lens 12 as its primary objective lens. The lens 12 could be refractive and, in the illustrated case, a double convex lens, or it could be diffractive in the form of, for example, a Fresnel zone plate or the like. Light 10 from a scene is collected by the lens 12 and converged to its focal point F. The physical arrangement of the device is such that the location of the focal point or plane of the lens 12 corresponds to that of the detector 14 of the optical system which may be a focal plane array (FPA) or the like. Thus, the distance between the lens 12 and the FPA of the optical system is dictated by the focal length of the lens 12 which, in turn is dictated by its size and optical characteristics (both of which are fixed).
Referring to FIG. 1B of the drawings, a reflective telescope or sensor arrangement comprises a pair of mirrors: a first, concave mirror 16 and a second, (for example) planar mirror 18 mounted at an angle relative to the incident light path. Light (or other electromagnetic radiation) is collected by the concave mirror 16 and directed back to the planar mirror 18, which is oriented at an angle to cause the radiation to be directed and focussed to a point corresponding once again to the FPA of the device. It will be appreciated, of course, that the mirrors 16, 18 can be of any desired size, shape and/or orientation to achieve the required beam direction and focussing.
Referring to FIG. 1C of the drawings, in a third arrangement, known in the art as catadioptric, a first concave mirror 22 and a second, opposing convex mirror 20 are provided in the configuration shown. The concave mirror 22 is provided with a generally central aperture 23. Light 10 is collected by the portion of the concave mirror 22 around the aperture 23 and reflected and converged toward the convex mirror 20. The light is then reflected back by the convex mirror 20 and converged to its focal point F, once again corresponding to the location of the FPA of the optical system.
Other configurations of telescope and long range sensors and image capture devices are known, having varying configurations of optical devices to capture and then focus electromagnetic radiation to a focal plane on which is located an imaging detector, for example, a focal plane array (FPA) for sensing. In all cases, the angular resolution is dependent on the focal length achievable. Focal length is defined as the distance between the centre of a lens or curved mirror and its focal point, or the equivalent distance in a compound lens or telescope. The larger the focal length, the greater will be the angular resolution of the device. Equally, the larger the primary objective lens or mirror (or other optical device used to ‘collect’ electromagnetic radiation), the better the quality of the sensed signal will be. In other words, the larger the collecting device, the better will be the quality of the collected signal, and the greater the focal length, the greater will be the magnification of the collected signal. Thus, the overall quality of the system is primarily dependent on the size of the collecting device and the focal length of the optical system.
However, in conventional sensor systems, the size of the primary objective lens and the focal length of the optical system are fixed and constrained by the physical apparatus in which it is mounted and the size of the lens which can be provided therein. Thus, the optical properties and capabilities of conventional sensors are fixed and limited by physical constraints. On the other hand, there is an ongoing desire to increase the range and angular resolution of telescopes and other long range sensors, which can currently only effectively be achieved by increasing the size of the overall apparatus, which can be undesirable in many applications and, in others, simply not practicable. It is an object of aspects of the present invention to at least address these issues.